The purpose is to compare the patterns of vocal fold vibration and the impairments in phonatory functioning found in patients with voice disorders of unknown etiology with those of patients with known neuropathologies affecting phonation to determine whether particular neuropathologies are implicated in phonatory disorders of unknown etiology. Vocal fold vibration and phonatory functioning is being analyzed in patients with spastic dysphonia and vocal tremor and compared with those of patients with known neuropathologies of the recurrent laryngeal nerve, and lower motor neuron and upper motor neuron disorders affecting laryngeal functioning. Laryngeal reflexes and respiratory functioning are being evaluated in these groups to determine the integrity of phonatory functioning. The effects of manipulating phonatory functioning through alterations in auditory feedback is being evaluated for identifying particular subtypes of these disorders which are predictive of the effects of different treatment approaches.